To Default the accounts, the Requisition Account Generator workflow - POWFRQAG would be called.
It is normal, that POWFRQAG does not default charge account for project based requisition.
You are required to customize the POWFRQAG workflow.
Details documented in the note:
Error APP-PO-14040 in Po Requisition Account Generator (Doc ID 263850.1)
There are many organization who uses auto accounting lookup sets to setup the account codes that will be used by project based requisitions/purchase orders.
Responsibility: Project Billing Super User
Setup -> AutoAccounting -> Lookup Sets
You may give it a go.